1. Field of the Invention
The present invention relates to a wavelength division multiplexing (WDM) optical network. More particularly, the present invention relates to an optical cross-connect (OXC) device for connecting optical networks.
2. Description of the Related Art
Recently, practical use has been achieved of a wavelength division multiplexing (WDM) technique adapted to transmit optical signals of different wavelengths through a single-core optical fiber. In additional, it is also now possible to transmit a larger quantity of data at high speed than previously known. Furthermore, the optical settings or switching travel paths of optical signals has been made possible in accordance with development of certain optical element techniques. As a result, the actual construction of an optical network based on the WDM technique is now possible. For such a WDM optical network, a bi-directional OXC device is used that includes a main unit comprising a pair of wavelength division multiplexers/demultiplexers and an optical switch, in which is similar to a general uni-directional OXC device, and an accessory unit used for separately processing forward and backward optical signals.
For each wavelength division multiplexer/demultiplexer there is an arrayed-waveguide grating (AWG) used to achieve an easy optical signal channel extension and a simple control, yet having a high degree of integration. For the optical switch, a 2×2 space switch is mainly used. In addition, for each accessory unit adapted to process bi-directional optical signals, there are 3-port elements such as circulators, WDM filters, or wavelength interleavers.
FIG. 1 is a block diagram illustrating the configuration of a conventional WDM optical network. As shown in FIG. 1, the WDM optical network includes first and second WDM ring networks 110 and 120 each having a plurality of nodes (not shown) connected to one another by optical fibers. Additionally, an optical cross-over device (OXC) 130 enables communication between the first and second WDM ring networks 110 and 120.
Each of the first and second WDM ring networks 110 and 120 perform intra-network communications using a forward optical signal comprising the first through n-th channels λ11 to λ1n, and a backward optical signal comprising first through n-th channels λ21 to λ2n. Here, “n” is a natural number. The first one of two reference numerals suffixed to “λ” represents a channel designations of the advancing direction of an optical signal associated with the channel. Moreover, the second reference numeral designates the order of the channel. For example, “λ12” designates the second channel of a forward optical signal, whereas “λ23” designates the third channel of a backward optical signal.
The OXC device 130 includes first and second external ports 131 and 132, EP1 and EP2 that are connected to the first ring network 110, and third and fourth external ports 133 and 134, EP3 and EP4 that are connected to the second ring network 120. The OXC device 130 supports intra-network communication for each of the first and second ring networks 110 and 120. Additionally, the OXC device supports inter-network communication between the first and second ring networks 110 and 120. For example, the OXC device 130 outputs a forward optical signal, which is input thereto at the first external port 131, to the second external port 132 or fourth external port 134, while outputting a backward optical signal, which is input thereto at the fourth external port 131, to the first external port 131 or third external port 133.
Also, the OXC device 130 can perform a switching operation for the forward and backward optical signals in the unit of channels. For example, the OXC device 130 may output, to the second external port 132, the first through m-th channels λ11 to λ1m of the forward optical signal which is input to the first external port 131, while outputting the (m+1)-th through n-th channels λ 1(m+1) to λ1n of the forward optical signal to the fourth external port 134. Here, “m” is a natural number not more than “n”.
FIG. 2 is a diagram illustrating a configuration of the OXC device shown in FIG. 1. As shown in FIG. 2, the OXC device 130 includes first through fourth wavelength selective couplers (WSCs) 141 to 144, that is, WSC1 to WSC4.
Further, there are 8 wavelength division multiplexers/demultiplexers (WDMs) 151 to 158, that is, WDM11 to WDM24, which are divided into first and second groups, each comprising first through fourth WDMs. There is also a plurality of switches (SWs) 161 to 166, that is, SW11 to SW2n, divided into two groups, that is, first and second groups, each consisting of first through n-th SWs.
With regard to certain designations, the “WDMxy” designates the y-th WDM of the x-th WDM group, and “SWxy” designates the y-th SW of the x-th SW group. For example, “WDM12” designates the second WDM of the first WDM group, whereas “SW23” designates the third SW of the second SW group. Each WSC has three ports, whereas each SW has four ports. Where it is assumed that one WSC or one SW is designated by a reference numeral “###”, its n-th port is designated by “n” in the drawings while being designated, in the following description, by a reference numeral “###n”. Also, it is assumed that the nn-th channel has an nn-th wavelength.
As shown in FIG. 2, the respective first ports 1411 to 1441 of the WSC1 and WSC4 141 to 144 are connected to the first through fourth external ports 131 to 134. Respective ports 1412 to 1442 of the WSC1 to WSC4 141 to 144 provide passages for forward optical signals, whereas respective ports 1413 to 1443 of the WSC1 to WSC4 141 to 144 provide passages for backward optical signals. The WSC1 and WSC3 141 and 143 output respective forward optical signals, which is input thereto at their first ports 1411 and 1431 and to their second ports 1412 and 1432, while outputting respective backward optical signals which is input thereto at their third ports 1433 and 1443 to their first ports 1411 and 1431.
On the other hand, FIG. 2 also shows the WSC2 and WSC4 142 and 144 output respective backward optical signals, which is input thereto at their first ports 1421 and 1441, to their third ports 1423 and 1443, while outputting respective forward optical signals, which is input thereto at their second ports 1422 and 1442, to their first ports 1421 and 1441.
Each of the WDM11 to WDM24 151 to 158 has only one multiplexing port, and n demultiplexing ports. The WDMs operate to demultiplex an optical signal which is input thereto at the multiplexing port in the channel unit. Respective channels of the demultiplexed optical signal are output to the demultiplexing ports, while multiplexing channels which is input thereto at the demultiplexing ports output the resultant optical signal to the multiplexing port.
For example, each switch of the SW11 to SW1n 161 to 163 in the first SW group has first through fourth ports: 1611 to 1614 in the case of SW11 161; 1621 to 1624 in the case of SW12 162; or 1631 to 1634 in the case of SW1n 163. Each switch is switched between a “bar” state, in which the first and second ports are connected to each other, and the third and fourth ports are connected to each other, and a “cross” state in which the first and fourth ports are connected to each other, and the second and third ports are connected to each other. For example, in the case of the SW11 161, its first and second ports 1611 and 1612 are connected to each other, and its third and fourth ports 1613 and 1614 are connected to each other in the bar state. Moreover, first and fourth ports 1611 and 1614 are connected to each other, and the second and third ports 1612 and 1613 are connected to each other in the cross state. Each switch in a respective SW group is connected at a first through fourth ports to respective corresponding demultiplexing ports of associated ones of the WDMs in the WDM group corresponding to the SW group.
For example, the SW1n 163 is connected at its first and third ports 1631 and 1633 to respective n-th demultiplexing ports of the WDM11 151 and WDM13 153. The SW1n 163 is also connected at its second and fourth ports 1632 and 1634 to respective n-th demultiplexing ports of the WDM12 152 and WDM14 154. Also, the SW2n 166 is connected at its first and third ports 1661 and 1663 to respective n-th demultiplexing ports of the WDM21 155 and WDM23 157. The SW2n is also connected at its second and fourth ports 1662 and 1664 to respective n-th demultiplexing ports of the WDM22 156 and WDM24 158.
Now, operation of the above-mentioned OXC device 130 will be described in detail in conjunction with the first case of outputting the channel λ11, which is input to the first external port 131, to the fourth external port 134, and the second case of outputting the channel λ22, which is input to the second external port 132, to the third external port 133.
The SW11 161 and SW22 165 are first set to be in their cross state by a control unit (not shown). In this case, the WSC1 141 outputs the channel λ11, which is input thereto at its first port 1411, to its second port 1412 to which the WDM11 151 is connected at its multiplexing port. The WDM11 151, which outputs the channel λ11 is inputted thereto at its multiplexing port to its first demultiplexing port to which the SW11 161 is connected at its first port 1611. The SW11 161 outputs from its third port 1613 the channel λ11 that is input thereto at its first port 1611 to which the WDM14 154 is connected at its first demultiplexing port. The WDM14 154 outputs the channel λ11 that is input thereto at its first demultiplexing port, to its multiplexing port to which the WSC4 144 is connected at its second port 1442. The WSC4 144 outputs the channel λ11, which is inputted thereto at its second port 1442, to its first port 1441.
Next, the second case will be described in detail. In this case, the WSC2 142 outputs the channel λ22, which is input thereto at its second port 1422, to its third port 1423 to which the WDM24 158 is connected at its multiplexing port. The WDM24 158 outputs the channel λ22, which is inputted thereto at its multiplexing port, to its second demultiplexing port at which the SW22 165 is connected to fourth port 1654. The SW22 165 outputs the channel λ22, which is input thereto at its fourth port 1654 to a first port 1651 to which the WDM21 155 is connected at its second demultiplexing port. The WDM21 155 outputs the channel λ22, which is input thereto at its second demultiplexing port, to a multiplexing port to which the WSC1 141 is connected at its third port 1413. The WSC1 141 outputs the channel λ22, which is input thereto at its third port 1413, to its first port 1411.
However, the conventional OXC device 130, which uses a plurality of WDMs and a plurality of 2×2 space switches, as mentioned above, has a problem of high manufacturing costs because the WDMs are expensive. Furthermore, the OXC device 130 requires a complex switching procedure because demultiplexing, switching, and multiplexing operations should be carried out upon switching one input channel. In addition, the OXC device 130 has a low channel extensibility because although the number of processible channels may be increased in accordance with an increase in the number of demultiplexing ports, in some cases it may be necessary to completely replace the existing WDMs with appropriate new ones appropriate for usage.